The present invention relates in general to communication systems and components therefor, and is particularly directed to a non-propagating magnetic field-based communication system having a reduced hardware complexity magnetic field generator and detector arrangement, in combination with a frequency shift keyed (FSK) modulation scheme. The present invention is configured to facilitate the transmission and reception of digital data within a limited coverage area environment, between a compact transmitter unit, such as that contained in an xe2x80x98trackingxe2x80x99 tag affixed to an object, and a digital detector/demodulator unit.
Although a variety of communication systems employ propagating magnetic fields, those which used non-propagating magnetic fields generated about a coil are less prevalent. As a non-limiting example, non-propagating magnetic fields may be employed in theft detection systems of the type installed in retail stores. Many of these systems, such as may be installed at the entry/exit of a retail establishment, are designed to convey only a single piece of dataxe2x80x94the presence of a xe2x80x98taggedxe2x80x99 item. While others, such as xe2x80x98smartxe2x80x99 card systems, may convey more than one bit, the amount of information they are capable of transmitting and detecting is still relatively limited.
The present invention is directed to a non-propagating magnetic field based communication system, that is configured to provide for simplex digital communications without restriction to the amount of data that may be transmitted, via an FSK-modulated non-propagating magnetic field emanating from a modulating source and sensed by an associated demodulating receiver. As a non-limiting example, the invention may be employed in a real time location system for locating and/or identifying transponder-tagged objects.
Pursuant to the present invention, the system employs an FSK transmitter unit having an analog section that generates and FSK-modulates the non-propagating magnetic field, and a digital section that converts incoming digital data into switch control signals. The switch control signals controllably switch capacitor components in circuit with a magnetic field coil, thereby modulating or changing the resonant frequency of an inductor-capacitor (LC) tank circuit, to effect FSK-modulation of the magnetic field in accordance with the digital data.
The magnetic field coil is small compared to the volumetric extent of its generated magnetic field, so that energy in the magnetic field is not propagated. Under supervisory digital control of a zero-crossing detector, that is coupled in parallel with the resonant LC tank circuit, a pumping switch is periodically operated in a fly-back manner, to provide a DC current boost to the magnetic field coil from its DC power supply, thereby compensating for resistive losses in the tank circuit. The pumping signal has a duration for a small fraction of a cycle of the resonant frequency of the magnetic field, and may be optimized for the intended range of operation of the generated field and the size of the coil.
Zero crossing points of the resonant frequency signal are supplied to a microcontroller for control of capacitor insertion switches of a multi-capacitor circuit, producing FSK modulation of the resonant magnetic field. During a calibration mode, vernier adjustment capacitors may be controllably switched in and out of the resonator tank circuit to determine optimum frequency matches for a desired FSK frequency pair. Thereafter, during actual data transmission, calibration-based xe2x80x98best matchxe2x80x99 capacitors are switchably inserted in parallel with a base capacitor, to precisely define a pair of resonant frequencies associated with the binary states of the digital data. To FSK modulate the magnetic field, a data spreading code, such as a Manchester or other relatively short spreading code used for reduced complexity data communications, may be employed.
An alternate embodiment of the transmitter unit eliminates the multi-capacitor circuit and employs a microcontroller to generate and control pulse timing and duration used to pump the field coil. This approach requires accurate values of inductance and capacitance in the resonant circuit, but offers the advantage of reduced parts count, allowing its use in compact, portable applications. Since the microcontroller pumps the circuit every cycle, frequency error due to resonant circuit tolerance is pulled into correction on a cycle by cycle basis.
The voltage supplied to the fly-back configuration also offers a suitable power control mechanism. This approach is favored for large changes in power, as it allows the pulse width of the pump to be maintained at the proper width for high efficiency. A variable voltage regulator may be employed to effect this change. Also, the use of the variable voltage regulator affords inclusion of a power control loop by monitoring the voltage produced in the resonant circuit and adjusting the supply voltage to maintain it at a constant level. This provides constant communication performance when large metal objects such as automobiles or forklifts move in close proximity to the transmitting unit.
The receiver unit includes an LC tank detector circuit that includes a magnetic field-sensing coil in parallel with an associated capacitor. The LC tank circuit resonates at a frequency between the two FSK frequencies employed by the transmitter unit. The resonant detector circuit is coupled to a sense amplifier, which amplifies the voltage produced by the tank circuit for the desired receiver sensitivity and buffers the detected voltage to the appropriate logic level for use by a digital receiver/demodulator.
The digital receiver is referenced to a clock frequency that corresponds to the difference between the FSK frequencies of the selected modulation pair. The digital receiver contains two signal buffer paths, that operate on alternate sample periods, corresponding to one-half the period of the received data spread code, so that at least one of the two buffer paths will not be sampling data during transitions in the received FSK frequency. The output of the sense amplifier is coupled to the clock input of a frequency counter, whose contents are coupled to data inputs of first and second selectively enabled alternate sample latches. The count value in the frequency counter is cleared upon active reset, or when its sample enable input is not active. When enabled, the frequency counter is incremented by the rising edge of the change in the output of amplifier. At the end of the sample time, the contents of the frequency counter are clocked into one of the two latches, whose contents are clocked into the other latch.
Since the contents of a respective latch indicate the number of successive rising edges of the received signal within a prescribed measurement interval (sample time), they are representative of the frequency of the latched data. This count value is coupled to the digital demodulator and compared with each of two stored counts associated with the two valid FSK frequencies. If the latched count is representative of a valid frequency, it is transferred to the other latch for subsequent comparison with the next frequency-associated count. The difference between the two latched count values is coupled to a state machine, which demodulates the spreading code of the data. The demodulated data is buffered, so that it may be clocked out for validation of parameters such as preamble, cyclic redundancy check (CRC) code sequence and message length.
The state machine demodulates the data by comparing successive FSK tones with a predefined start-of-message sequence. Upon detecting this sequence, the state machine initializes the data demodulation circuitry, so that the data may be clocked out as it is detected and demodulated. As is customary in FSK-modulation systems, data values may be represented by respectively different sequences of the two FSK tones. Similar to detecting the start of a message, the state machine may detect the end of a message by comparing successively received FSK tones with a predefined end-of-message sequence. Upon detecting a valid end-of message sequence, the state machine returns the receiver""s demodulation circuitry to its idle state.